Prior arrangements of web drying apparatus of the general type to which this invention relates are disclosed in U.S. Pat. No. 3,873,013, issued to P. H. Stibbe in 1975 and U.S. Pat. No. 3,739,491, issued to R. W. Creapo et al in 1973, both of which have a common assignee with this application. All such apparatus provides for contactless support of a freshly imprinted or freshly coated web of paper or the like as it moves lengthwise through a dryer enclosure. The web is oriented horizontally and is floated on air streams that issue against it from a plurality of air bars arranged along its path, each extending across the width of the web. A row of air bars is located beneath the web to discharge air upwardly and levitate it, and another row is above the web, discharging downwardly to steady and guide it.
In the apparatus of each of the above mentioned patents, a single blower that supplied pressure air to all of the air bars was communicated with the air bars through a complicated and rather bulky ductwork system that was intended to cause the air flow emitted from each air bar to be uniform all along the length of it and to be substantially uniform with the air emissions from all of the other air bars. The blower delivered pressure air into one end of a delivery duct that extended parallel to the length of the web, in outwardly spaced relation to one edge of the web. At its opposite end the delivery duct discharged into a pair of elongated plenum chambers, each of which extended widthwise relative to the web, one above it, the other below it. From each of these plenum chambers a pair of headers projected along the web, in the direction opposite to that of air flow along the delivery duct. Each of the air bars below the web had connections to the two headers beneath the web, and the air bars in the row above the web were similarly connected with the two upper headers. The arrangement of the delivery duct and plenum chambers ensured a smooth, steady delivery of pressure air into each header, and each header tapered along its length to afford substantially uniform air flows at all of its air bar connections. Each air bar had its connections with its two headers at locations between its midpoint and its respective ends, to provide for discharge of an airstream that was substantially uniform all along the length of the air bar.
As stated in the introductory paragraph of Pat. No. 3,739,491, "Prior art dryers of this general character have been used with a certain degree of success but have had certain shortcomings such as frequently being of considerable size and thereby requiring excessive installation space." Elsewhere in the patent the apparatus that it discloses is described as "particularly compact" by reason of the blower being close to the air bar assemblies.
The above described arrangement probably represented the utmost in compactness that has heretofore been attainable, but it nevertheless required the dryer assembly to be substantially wider than the printing press with which the dryer was associated, inasmuch as the blower and its delivery duct had to be spaced edgewise outwardly from the web. In many press rooms floor space is limited, and while such floor space limitations tend to be more stringent in Europe than in the United States, it is always very desirable--even when not essential--that equipment occupy as little floor space as possible.
In the apparatus of each of the above mentioned patents only one blower supplied pressure air to all of the air bars. Although it would seem to be obvious that compactness is more readily achieved with a single blower than with a multiplicity of blowers, the present invention rejects the obvious in favor of employing a plurality of blowers, one for each row of air bars. The logic of the new arrangement is that one blower can be mounted above the web and one below it, so that the blowers are located within a zone defined by the width of the web. There is no reason for the plural small blowers to consume any more energy than the single large blower that they replace, and if their total cost is more than that of one large blower (which would not necessarily be the case) the difference will be more than offset by the value of the floor space conserved and by the lower cost of the more compact dryer structure.
A person skilled in this art will probably recognize immediately that a plural blower arrangement which will achieve optimum compactness widthwise of the web is one wherein there is a blower for each row of air bars and each blower discharges directly into an elongated header which in turn discharges directly into each air bar in its row. Such a person will also recognize immediately that the last described arrangement, in any of its obvious forms, is not likely to provide for uniform discharge of pressure air into the several air bars along the length of the header and is therefore likely to operate unsatisfactorily or not at all. Nevertheless, the present invention contemplates just such an arrangement, although, of course, in a form that ensures very satisfactory operation.
The present invention also achieves compactness in the direction lengthwise of the web, in that the blower for each header is located at the side of its header that is remote from the web, but the header nevertheless has outlet ports at its opposite side, at intervals all along its length, so that certain of those ports are directly opposite the blower. According to conventional wisdom, the air bars at those certain outlet ports would receive air from the blower at a substantially higher pressure, or with a substantially more turbulent or pulsing flow, than the air bars connected with other outlet ports.
The provision of a truly compact apparatus for contactless web support involves other problems that have no obvious solutions. To facilitate threading a web through the dryer, the air bars of at least one row should be retractable by movement away from the plane of the web. If the blower is to move for such retraction along with the rest of the air bar assembly, the substantial weight of the blower has to be lifted and controlled in descending motion and must be supported in the operative and retracted positions. Furthermore, there has to be space in the dryer enclosure to accommodate the blower as it moves to its retracted position. Providing space for blower movement may not be a troublesome problem when only a single web extends through the dryer, but in many cases two or more webs must extend through the dryer one above the other, requiring stacked arrays of air bars, and in such cases the need for vertical compactness may be as urgent as the need for horizontal compactness.